pH and chain length dependence of phase transition parameters of phosphatidic acid: experiment and theory

There is increasing evidence that phosphatidic acids (PA) play a key role in aggregation, fusion and transport processes in cellular membranes. The aim of this research was to investigate how does the variation in environmental pH and in chain length influence the phase transition properties of aqueous PA dispersions. The phase transition parameters (temperature, enthalpy) were measured by differential scanning calorimetry. Our experimental data are compared with those available in literature. A critical review is given too. From these data the phase transition parameters vs pH or chain length diagrams are constructed. The transition temperature and enthalpy decrease with increasing pH due to growing electrostatic repulsion in the head group region. The longer the apolar chains are, the higher are the transition parameters. The cooperative unit sizes in different conditions are calculated too. The behaviour of PA is compared to those of phosphatidylcholines and ethanolamines. A Landau-theory is used for the theoretical description of the chain length dependence of the transition parameters at different pH values.